Many power supplies today contain switches driving a transformer in order to efficiently convert either an unregulated voltage supply (B+) to one or more regulated output supplies or to shift the voltage level between a high voltage input and a low voltage output or a low voltage input to a high voltage output. These supplies are usually referred to as switch mode power supplies, and the switching drive, often called a chopper, is usually a semiconductor-- either a bipolar transistor or a field effect transistor (FET). It should be noted that the claimed invention is not limited in application to semiconductor choppers, but is generally applicable to any switch mode supply which can employ the use of negative feedback to maintain a constant output voltage.
A switch mode power supply differs from a classical power supply chiefly in the use of these chopper switches which can operate at frequencies significantly higher than the typical power line frequency of 50 or 60 hertz. Once this higher frequency alternating current (a.c.) voltage is generated, typically at 8-40 kilohertz, a much smaller, lighter, and less expensive transformer can be employed than would be necessary for the same level of power conversion at 50 or 60 hertz.
One significant problem in any supply is the fact that the transformer itself should never have a net d.c. voltage impressed across its primary. If such a net d.c. voltage exists across the transformer primary, an enormous current can build up since the d.c. impedance of the transformer primary is only limited by the resistance of the primary windings themselves.
Unfortunately, in practice it is very difficult to perfectly balance the chopper in a switch mode supply, which leads to the creation of an unwanted net d.c. voltage on the transformer primary. Some sources of imbalance are:
1. Difference in the two halves of the transformer primary.
2. Differences in the saturation voltage of the driving switches.
3. Nonsymetry in the driving waveform.
4. Differences in the storage time of the driving switches.
The transformer primary circuit will automatically average over all such imbalances and a current spike will occur which will force the d.c. voltage on the transformer primary to zero. This spike in current is often referred to as "ratcheting".
Thus, at the end of the chopper cycle the transformer core will saturate due to a primary imbalance and the magnetizing current will abruptly rise toward infinity. At a high enough current level, an "on" chopper switch will come out of its "on" state resulting in significant heating in this supposedly "on" device. ("On" refers to a switch which is conducting current with a very low voltage drop, as opposed to an "off" switch which is in its nonconducting state. Semiconductor switches are also capable of a third "active" state, in which current will flow while the voltage across the switch is at some intermediate level between the two power terminals.) Thus, if the actual current spike does not itself damage the switch, the resulting internal heating will.
Various methods have been employed to reduce the ratchet effect based either on the use of larger magnetic cores or complex magnetic paths to prevent transformer saturation. These methods work, but they make the transformer significantly more complex and/or expensive. The disclosed invention is both inexpensive and easily implemented with the use of readily available electronic components and can practically eliminate the ratchet effect.